Servovalves in which a primary stage valve is used to control a secondary output control valve have been used extensively in the past. In such valves, the secondary output valve usually comprises a spool, spring-biased to a central, null position (or to the primary stage valve) which can be shifted in one direction or the other by differential pressure operating at opposite ends of the spool. The differential pressure, in turn, is controlled by a primary stage valve which varies the back pressure generated by two nozzles which direct jets onto the primary stage valve. It is these back pressures which act on the spool to move it in one direction or the other from its central, null position.
In the past, it has been common to control the position of the primary stage valve with an analog torque motor. Such torque motors, however, are subject to null shift or flow gain changes which adversely affect the operation of the valve. Furthermore, the electronic systems controlling the torque motors have a tendency for hardover failures.